External defibrillator

ABSTRACT

An external defibrillator estimates the phase of ventricular defibrillation (VF) by deriving, from an ECG exhibiting VF, at least one quality marker representing the morphology of the ECG and, therefore, the duration of the VF. The duration of the VF is calculated as a function of the value (s) of the quality marker (s). The quality marker (s) may comprise any one or more of the median slope of the ECG, the average slope of the ECG, the ratio of the power in relatively high and low frequency bands of the ECG, and a measure of the density and amplitude of peaks in the ECG, over a predetermined period.

FIELD OF THE INVENTION

This invention relates to an external defibrillator.

BACKGROUND

Following the publication of a three-phase time sensitive model byWeisfeldt and Becker (Weisfeldt M L, Becker L B: “Resuscitation aftercardiac arrest. A 3-phase time-sensitive model”; JAMA. 2002; 288:3035-3038), much research has focused on developing treatment algorithmsspecific to each of three phases of cardiac arrest.

The first phase is known as the “Electrical Phase” and constitutes thefirst four minutes of a cardiac arrest. During this time, immediatedefibrillation should be administered.

The second phase is known as the “Circulatory Phase” and occurs afterthe first phase for another period of four minutes—that is, four to tenminutes after arrest. During this phase, CPR should be administeredbefore defibrillation in order to increase perfusion and prepare themyocardium for defibrillation by re-oxygenation, thereby increasing thechances of success of the therapy.

The final phase is known as the “Metabolic Phase” and the only availabletreatments are mild or moderate hypothermia, metabolic therapies or theuse of Caspase inhibitors, all of which are only applicable toin-hospital patients.

Studies have shown that survival rates are much lower for patientspresenting prolonged ventricular fibrillation (VF). In these cases,immediate defibrillation appears to simply convert the patient'selectrocardiogram (ECG) from one non-perfusing rhythm (i.e. VF) toanother (i.e. PEA/asystole). It has also been shown that immediatedefibrillation in cases of prolonged cardiac arrest would result fromcountershock-induced injury to ischemic myocardium.

The condition of the myocardium deteriorates rapidly without effectiveCPR to perfuse the heart muscle and other vital organs. It is widelyaccepted that, for VF of short duration (less than 4 mins since VFonset), immediate shock therapy is indicated, whereas for VF of longduration (more than 4 mins since VF onset), CPR prior to defibrillationincreases the chances of return of spontaneous circulation (ROSC).

It is evident that if the responder had accurate information as to whichphase of VF the patient was presenting, they could deliver the mostappropriate form of therapy and improve their chances of survival.

SUMMARY OF THE INVENTION

According to the present invention there is provided an externaldefibrillator comprising means for estimating the phase of ventricularfibrillation (VF) by analysis of the patient's ECG and means dependenton the estimated phase for indicating whether an immediate shock or CPRis advised.

In certain embodiments the means for estimating the phase of VFcomprises estimating VF duration and comparing the estimated durationwith a threshold level. In such a case VF duration is preferablyestimated by deriving at least one VF quality marker from a patient'sECG and calculating the duration of VF as a function of the value(s) ofthe quality marker(s).

The quality marker may comprise the median slope of the ECG over apredetermined period, the average slope of the ECG over a predeterminedperiod, the ratio of the power in relatively high and low frequencybands of the ECG over a predetermined period, or a measure of thedensity and amplitude of peaks in the ECG over a predetermined period.

In another embodiment the means for estimating the phase of VF comprisesderiving a quantity related to the density and amplitude of peaks in theECG over a predetermined period.

In such case said quantity is derived by constructing an envelope of theECG and measuring the average magnitude of peaks lying above theenvelope during the predetermined period. The said quantity ispreferably compared to a threshold level to estimate the phase of VF.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows how VF changes its morphology from its onset with time.

FIG. 2 shows the construction of an ECG envelope in one embodiment ofthe invention.

FIG. 3 is the flow diagram of an algorithm to calculate the envelope ofFIG. 2 for use in an embodiment of the invention.

FIG. 4 is a block diagram of an automated external defibrillatorembodying the invention.

FIG. 5 is a flow diagram of a further embodiment.

DESCRIPTION OF EMBODIMENTS OF THE INVENTION

FIG. 1 shows how VF changes its morphology over time from its onset. AnECG exhibiting VF is shown in its early stages shortly after onset(left) and after 12 minutes (right). It can be seen that at thebeginning of VF higher frequencies, higher amplitudes, higher slopes anda greater concentration of peaks are found. These changes in VFmorphology reflect the worsening condition of the myocardium over timeduring uninterrupted VF.

The embodiments of the invention are based on the use of so-called“quality markers” for VF. In the present context a VF quality marker isa parameter, derived from an ECG exhibiting VF, which represents themorphology of the ECG and, therefore, it changes with the duration ofthe VF.

The embodiments teach a system incorporated into an automated externaldefibrillator (AED) which measures the ECG of a patient using the twodefibrillator electrodes, calculates one or more VF quality markers andestimates, using an algorithm, the duration of the VF using the qualitymarker(s).

The prior art teaches two VF quality markers, referred to herein asFrequency Ratio (FR) and Median Slope (MS). In addition, two new VFquality markers are disclosed in the present specification, referred toherein as Density and Amplitude of Peaks (DA) and Average of Slopes(AS).

All, any or some of these quality markers can be used, in conjunctionwith the defibrillator's own diagnostic algorithm, to produce audibleand visible indications to the operator to perform CPR prior thedelivery of the shock in order to increase the chances of a successfulresuscitation.

The VF Quality Markers

A sequence of ECG samples x₀, x₁, . . . , x_(N−1) in a window of Nsamples at fs samples per second is processed in successive epochs inorder to obtain the following quality markers: Median Slope (MS),Average Slope (AS), Frequency Ratio (FR) and Density and Amplitude ofPeaks (DA).

The Median Slope

The median slope as a shock outcome predictor is disclosed byEilevstjoønn et. al. (Eilevstjønn J, Kramer-Johansen J, Sunde K: “Shockoutcome is related to prior rhythm and duration of ventricularfibrillation”; Resuscitation. 2007; 75 60-67).

The median slope (MS) is given by:

${MS} = {\underset{{i = 1},2,\ldots \mspace{14mu},{N - 1}}{median}\left\{ {\left( {{x_{i} - x_{i - 1}}} \right){fs}} \right\}}$

For simplicity, the slope here is denoted by x_(i)-x_(i−1) where twoconsecutive samples are used. However, many more points can be used forcalculating the slope. The slopes are scaled and then sorted for thecalculation of the median slope. By definition, the median is thecentral value of an array of N reordered samples. If N is odd, the valuecorresponding to the position (N+1)/2 in the array is the median.Otherwise (if N is even) the median is given by the semi-sum of thevalues in the array corresponding to the positions N/2 and N/2+1.

The Average Slope

The average slope (AS) is expressed by:

${AS} = {\frac{1}{N - 1}{\sum\limits_{i = 1}^{N - 1}\; {\left( {{x_{i} - x_{i - 1}}} \right){fs}}}}$

The Frequency Ratio

The Frequency Ratio (FR) as an indicator of VF duration is disclosed bySherman (Sherman LD: “The frequency ratio: An improved method toestimate ventricular fibrillation duration based on Fourier analysis ofthe waveform”; Resuscitation. 2006; 69: 479-486).

Sherman presented a method based on the frequency analysis of the VFwaveform. VF data was recorded for 12.5 minutes in 45 swine. The Fourierfrequency spectra were calculated for 5 second epochs. The average powerat each frequency showed a marked loss of frequencies above 8 Hzoccurring at 5 min accompanied by an increase in the power in frequencyspectra from 3 to 5 Hz. The Frequency Ratio was defined as the ratio ofthe power in the high frequency band from 8 to 24 Hz compared to thepower in the low frequency band from 3 to 5 Hz. The Frequency Ratio wasshown to detect 90% of epochs in VF less than 5 min while allowingselection of 74% of those epochs over 5 min. When the Frequency Ratiowas set to detect 90% of episodes of VF under 7 min, it was able toselect 88% of those traces with VF over 7 min. The receiver operatingcurve (ROC) for the frequency ratio had an area under the curve of 0.91at 5 min and 0.95 at 7 min of VF duration.

Sherman claims that the Frequency Ratio is a strong estimator of VFduration. However it is based on frequency analysis which iscomputationally costly especially for the proposed range of frequencies.

According to the procedure presented by Sherman, the sequence of samplesx₀, . . . x_(N−1) (in the time domain) is transformed into the sequenceof N complex numbers X₀, . . . , X_(N−1) (in the frequency domain) bythe Discrete Fourier Transform (DFT) according to the formula:

$X_{k} = {\sum\limits_{n = 0}^{N - 1}\; {x_{n}^{\frac{{- 2}\pi \; }{N}{kn}}}}$k = 0, 1, …  , N − 1 where $^{\frac{2\pi \; }{N}{kn}}$

is a primitive N^(th) root of unity.

Let ^(P) _(k)=X_(k) X _(k) the power at the frequency index k.

The low frequency band (3-5 Hz) is associated with the frequency indexesfrom I₁ to I_(n).

Then the power of the low frequency band is found by:

$P_{low} = {\sum\limits_{k = l_{1}}^{l_{n}}\; P_{k}}$

Similarly, the power of the high frequency band is given by:

$P_{high} = {\sum\limits_{k = h_{1}}^{h_{m}}\; P_{k}}$

where h₁ and h_(m) are the frequency indexes for the high frequency band(8-24 Hz).

Finally the Frequency Ratio (FR) is defined as:

FR=P _(high)/P_(low)

However, due to the fact that Fast Fourier Transforms (FFTs) are timeconsuming and use significant resources of the CPU, in the presentembodiment the estimation of the magnitudes for different frequencies iscarried out using integer filters—rather than analyse for allfrequencies, filter through those, at discrete known frequencies, whichare known to make the most significant contribution. This technique isdescribed in our Irish Patent Application No. S2008/0785.

The Density and Amplitude of Peaks

For this quality marker an envelope of the ECG signal is used. At everysample it is checked if a peak is detected which is defined as anoutstanding value outside the envelope. The envelope is an artificialand auto adjusted signal created from the ECG signal in order to containit. However, an arriving sample, an outstanding one, can lie outside theenvelope. The principle in deriving the envelope is to construct a rayaiming at the baseline at a particular rate but the ray can be reset bya sample “obstructing” its path to the baseline.

FIG. 2 is an example of an envelope for an ECG signal during VF. Theenvelope 10 “contains” the signal 12 providing an estimation of themaximum value of the signal at a particular instant. This (local)maximum value or peak aims at the baseline and is reset as the amplitudeof the signal increases. As mentioned, certain peaks 14 may lie outside(i.e. above) the envelope.

In FIG. 3 the flow diagram for the calculation of the envelopecorresponding to each sample is presented. An explanation of thevariables used and their initial values in digital units is presented inthe following table:

Initial Variable Description value sample: ECG sample that feeds thealgorithm 0 at a rate of 170.6 samples/s abs_sample: Absolute value ofsample. 0 envelope: Artificially created signal that acts 5000 uV as aray aiming to the baseline. In its path to the baseline it can beinterrupted by an outstanding sample that reset its height. max_peak:Prospective value that potentially 5000 uV “climb” to an expectedmaximum. last_peak: Keeps a record of the value outside 5000 uV theenvelope encountered. seeker_decrement: Value taken from the peak_seeker  8 uV to give the following value for the peak_seeker. clearance:Counter to control when max_peak can 0 be updated. rate Sample rate(samples per second) 170.6 

The parameters used in FIG. 2 and in the table above were used with asample rate of 170.6 samples per second. However the parameters maychange according to changes in the sample rate.

The Density of Peaks quality marker is given by:

${DA} = {\frac{1}{N}{\sum\limits_{i = 0}^{N - 1}\; w_{i}}}$

where: w_(i)=|x_(i)| if x_(i) is a peak lying outside (i.e. above) the envelope, otherwise w_(i)=0.

If desired, the formula for DA can be multiplied by fs (the samplerate), as was done for the AS and MS quality markers. As fs is aconstant, the effect of this multiplication will be just scaling but theoriginal concept remains unchanged.

VF Duration

In order to compute an estimate of the VF duration from these markers,the following model is used:

$t = {{- B} + {C\left\lbrack {- {\ln \left( \frac{Q - D}{A} \right)}} \right\rbrack}^{E}}$

where t is the estimated VF duration in seconds, A,B,C,D and E areparameters and Q is the value of the quality marker.

Q may represent more than one quality marker, and in general a linearcombination of the marker values may be used:

Q=p(AS)+q(MS)+r(DA)+s(FR)

where p, q, r and s are coefficients (scaling factors).

A, B, C, D, E, p, q, r and s are empirically derived from a database ofECG signals during VF known to represent a range of ECG qualities. For aparticular ECG signal, a quality marker is calculated at every sample.Using the quality marker, an estimation of t is attempted and acomparison to the real time for VF duration is carried out. The valuesfor the parameters and coeficients are iteratively adjusted with the aimof minimising the difference between the estimated VF duration and thereal one.

The following are the likely ranges of the constants A, B, C, D and E:

A=+1 to +1000

B=−500 to +500

C=+1 to +500

D=0 to +200

E=+1/2 to +5/2

Defibrillator Hardware

FIG. 4 is a block diagram of an automated external defibrillatorembodying the invention.

The hardware used in the embodiment is standard to an automated externaldefibrillator and involves the measurement of the ECG potentials throughthe two electrodes D1 which also serve to deliver the shock therapy whenrequired.

In use, the patient's ECG is sensed by the defibrillator electrodes D1and passed to a differential amplifier D2. The latter is protected bycircuitry D11 from the high voltage which, if required by the patient,is applied to the defibrillation electrodes D1 during electro-therapy.The resultant signal (±3 mV) is passed to a first-order high pass filterD3 which, by means of feedback into the differential amplifier D2,restores the DC level to zero and removes the effects of respiration andmovement (below 1.6 Hz). The resultant signal is passed to afourth-order low pass filter D4 to remove mains pick-up and any otherhigh frequency noise (above 20 Hz). Finally, the signal is scaled in anamplifier D5 to a level required by a microprocessor D6 foranalogue-to-digital conversion and sampling. In this embodiment suchsampling occurs at a rate of 170.6 samples per second.

The microprocessor D6 not only analyses the ECG signal, using knowntechniques, to determine whether or not the patient is in ventricularfibrillation, but controls indicators D7 which guide the user in thedelivery of the electro-therapy. The indicators may comprise voiceprompts and/or coloured lamps which are illuminated to indicatepredetermined conditions.

On detecting VF, the microprocessor D6 automatically initiates a VFduration algorithm (to be described below) to estimate the current phaseof the VF. Depending on the outcome, the defibrillator will eitheradvise the application of CPR (to increase the quality and thereforelikely effectiveness of any shock) or advise immediate shock. When ashock is advised, whether immediate or after CPR, charging of capacitorsD9 will be initiated by the microprocessor D6 through activation of thecharge circuit D8. The voltage on the capacitors is sensed by themicroprocessor and, when at the correct energy level to be applied tothe patient, a bridge D10 is activated to apply a biphasic shock to thepatient when a “Shock” button (not shown) s pressed by the user.

It should be noted that the hardware shown in FIG. 4 is largely standardto all automated external defibrillators. The difference from theconventional machines is the ability to estimate the duration of the VFand to advise the user via voice prompts and/or coloured lamps D7 as towhether immediate electro-therapy or CPR followed by electro-therapy isadvised. This is achieved using a novel algorithm embedded in themicroprocessor software.

Defibrillator Software

The steps of the software algorithm are:

If VF is detected:

-   -   a. Determine the value Q of the quality markers, or such of them        as are used, for the preceding 4-second epoch of the ECG.        Thereafter, for each digital sample, update the value Q for the        immediately preceding 4-second epoch of the ECG. After the        initial calculation of Q the calculation is an updating process        rather than a full calculation, in order to keep processing time        to a minimum. This ensures both avoid CPU overload and the        minimum delay in advising or administering the correct therapy.        In the present embodiment only the markers AS and DA are        calculated, and the value Q is given by:

Q=0.75(AS)+0.25(DA)

-   -   b. At certain predefined intervals, e.g. every 1 sec, for the        current value of Q estimate the duration t of VF from its onset        using the foregoing model. In this embodiment the following        values were used in the model:

A=480

B=0

C=180

D=85

E=+1/2

-   -   c. Determine the phase of VF. VF is deemed to be in the        Electrical Phase if t is lower than 240 seconds (4 min).    -   d. Recommend action to the user by audible and visual guidance        and advice via the indicator(s) D7:        -   If VF is in the Electrical Phase, immediate shock therapy is            recommended and the defibrillator shock circuits are            enabled.        -   If VF is not in the Electrical Phase, CPR is recommended            prior to shock therapy. The defibrillator shock circuits are            not enabled until it is recommended that CPR be given, but            may be manually overridden.

FIG. 5 is an alternative algorithm for determining the VF phase in thecase of the DA quality marker. As before, for each 4-second epoch theenvelope 10 is calculated as described with reference to FIGS. 2 and 3,step 100. In steps 110 and 112 the DA quality marker is determinedaccording to the formula for DA given earlier as the average of theabsolute values of the samples in the epoch that lay above the envelope.

Next, however, a direct estimation of the VF phase is made by comparingthe value of DA with a predetermined threshold level, step 114. In thepresent embodiment this threshold is 0.29 mV/sample, and if DA is equalor greater than this threshold VF is assumed to be in the ElectricalPhase. Thus immediate shock therapy is recommended and the defibrillatorshock circuits are enabled.

If DA is less than the threshold, it is assumed that VF is not in theElectrical Phase. Thus CPR is recommended prior to shock therapy. Thedefibrillator shock circuits are not enabled until it is recommendedthat CPR be given, but may be manually overridden.

It will be seen that this embodiment avoids the need to estimate theduration of VF.

The invention is not limited to the embodiments described herein whichmay be modified or varied without departing from the scope of theinvention.

1. A method comprising: receiving electrical signals from the pair ofelectrodes when the pair of electrodes are in contact with a patient;determining, via a processor and based on the electrical signals, anelectrocardiogram of the patient; and when, based on theelectrocardiogram, the patient is in a state of ventricularfibrillation: identifying a density of peaks over a period of time inthe electrocardiogram; estimating a current phase of ventricularfibrillation based on the density of peaks; and outputting aninstruction based on the current phase.
 2. The method of claim 1,wherein estimating of the current phase of ventricular fibrillationfurther comprises comparing the density of peaks to an envelope.
 3. Themethod of claim 2, wherein the density of peaks is calculated based on anumber of peaks which exceed a threshold of the envelope within theperiod of time.
 4. The method of claim 3, further comprising modifyingthe threshold of the envelope based on an average peak height over aprevious period of time.
 5. The method of claim 1, wherein theestimating of the current phase of ventricular fibrillation is furtherbased on an average of slopes over the period of time.
 6. The method ofclaim 5, wherein the estimating of the current phase of ventricularfibrillation is further based on a median slope over the period of timeand a frequency ratio over the period of time.
 7. The method of claim 1,wherein outputting of the instruction comprises outputting an audiblesignal.
 8. The method of claim 7, wherein the audible signal indicatesthe patient is not in an electrical state of ventricular fibrillationand cardiopulmonary resuscitation should begin.
 9. The method of claim1, wherein the duration of ventricular fibrillation as a function of thevalue(s) of the quality marker(s) is calculated as:$t = {{- B} + {C\left\lbrack {- {\ln \left( \frac{Q - D}{A} \right)}} \right\rbrack}^{E}}$where t is the duration of the ventricular fibrillation in seconds,A,B,C,D, and E are constants, and Q is the value of the quality marker ,if only one, or a linear combination the values of the quality markersif more than one.